This section is intended to provide a background to the various embodiments of the invention that are described in this disclosure. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In the 3rd Generation Partner Project (3GPP) Long Term Evolution (LTE) system, Multiple Input Multiple Output (MIMO) techniques, such as open-loop and close-loop spatial multiplexing, are introduced to improve spectrum efficiency and network capacity. For these transmission modes (e.g. TM3, TM4 and TM8 in 3GPP Release-9), single layer or multiple layers can be applied based on a radio channel's quality and rank.
Rank generally represents the number of transmission layers for wireless communication between a base station (e.g., an evolved Node B (eNB)) and a user equipment (UE) in a MIMO system. Taking rank 1 and rank 2 downlink transmission as an example, the rank 1 represents that the base station transmits downlink data to the UE by using a single layer, where the same data is transmitted via two different transmitting antennas of the base station to achieve space diversity; while the rank 2 represents that the base station transmits downlink data to the UE by using two different layers via the two different transmitting antennas to achieve space multiplexing.
Rank adaptation relates to flexibly selecting, from a plurality of ranks allowed by the corresponding transmission mode, a rank for wireless communication between the base station and the UE. Throughput for the UE would be maximized if the rank is selected properly at the eNB side to match well with the real channel condition. Usually, the eNB may determine the rank by several approaches, some of which are described in the following to introduce aspects that may facilitate a better understanding of the embodiments of the invention described later in this disclosure.
In one approach, the eNB may simply follow rank indication (RI) included in the channel state information (CSI) report received from the UE so as to perform data transmission. As an example of such approach, Chinese Patent Publication No. CN 101998498A, entitled “Rank self-adapting method and rank self-adapting device”, discloses a rank self-adapting solution, where a rank receiving unit is used for receiving reported ranks of UE within a rank receiving window; a rank decision unit is used for determining a rank of the current moment according to the distribution state in the rank receiving window and determining the number of independent channels for downlink data transmission according to the determined rank of the current moment. In another approach, the eNB may determine a rank according to a suitable parameter indicative of channel quality. For example, rank 1 is adopted if estimated channel quality is below a certain threshold, otherwise rank 2. The parameter indicative of channel quality can be derived from the CQI report received from the UE or outer-loop link adaptation results according to configured rules by the communication system. As an example of such an approach, Chinese Patent Publication No. CN 102035580A, entitled “Method for retreating rank in spatial multiplexing mode, base station and communication system”, discloses a rank adaptation solution for retreating rank in a spatial multiplexing mode where a base station receives CQI and RI from a terminal; and when the RI is larger than 1, according to the CQI and preset threshold information, judging whether a downlink channel goes through deep fading with small scale, and if so, using a degraded value than the RI reported by the terminal as the current rank used by the base station. In still another approach, the eNB may estimate rank by sounding reference signal (SRS) or demodulation reference signal (DMRS) in uplink based on the uplink downlink channel reciprocity for a Time Division Duplex (TDD) system.
Classical Versus Shared Radio Cell Deployments
In the following, the term point is used to mean a point having transmission and/or reception capabilities. As used herein, this term may interchangeably be referred to as “transmission point”, “reception point” or “transmission/reception point”. To this end, it should also be appreciated that the term point may include devices such as base stations (e.g. eNBs) and radio units (e.g. Remote Radio Units (RRUs)). As is known among persons skilled in the art, base stations generally differ from RRUs in that the base stations have comparatively more controlling functionality. For example, base stations typically include scheduler functionality, etc., whereas RRUs typically don't. Therefore, RRUs are typically consuming comparatively less computational power than base stations. Sometimes, base stations may therefore be referred to as high power points whereas RRUs may be referred to as low power points. In some cell deployments, low power points are referred to as pico points and high power points are referred to as macro points. Thus, macro points are points having comparatively higher power than the pico points.
The classical way of deploying a network is to let different transmission/reception points form separate cells. That is, the signals transmitted from or received at a point is associated with a cell-id (e.g. a Physical Cell Identity (PCI)) that is different from the cell-id employed for other nearby points. Conventionally, each point transmits its own unique signals for broadcast (PBCH (Physical Broadcast Channel)) and sync channels (PSS (primary synchronization signal), SSS (secondary synchronization signal)). The classical way of utilizing one cell-id per point is depicted in FIG. 1 for a heterogeneous deployment where a number of low power pico points are placed within the coverage area of a higher power point. Note that similar principles also apply to classical macro-cellular deployments where all points have similar output power and perhaps are placed in a more regular fashion compared with the case of a heterogeneous deployment.
A recent alternative to the classical cell deployment is to instead let all the UEs within the geographical area outlined by the coverage of the high power point be served with signals associated with the same cell-id (e.g. the same Physical Cell Identity (PCI)). In other words, from a UE perspective, the received signals appear coming from a single cell. This is illustrated in FIG. 2. Note that only one macro point is shown, other macro points would typically use different cell-ids (corresponding to different cells) unless they are co-located at the same site (corresponding to other sectors of the macro site). In the latter case of several co-located macro points, the same cell-id may be shared across the co-located macro points and those pico points that correspond to the union of the coverage areas of the macro points. Sync channels, BCH (Broadcast Channels) and control channels and CRS (Cell Reference Signal) may all be transmitted from the high power point while data can be transmitted to a UE also from low power points by using shared data transmissions (e.g. a Physical Downlink Shared Channel (PDSCH)) relying on UE specific resources. Such an approach has benefits for those UEs that are capable of PDSCH based on UE specific resources while UEs only supporting CRS for PDSCH (which is likely to at least include all Release 8/9 UEs for Frequency Division Duplex (FDD)) has to settle with the transmission from the high power point and thus will not benefit in the downlink from the deployment of extra low power points. In FIG. 2, the high power point may be a base station such as an eNB. The low power points may be radio units such as those commonly referred to as Remote Radio Units (RRUs).
The single cell-id approach can be geared towards situations in which there is fast backhaul communication between the points associated to the same cell. A typical case would be a base station serving one or more sectors on a macro level as well as having fast fiber connections to remote radio units (RRUs) playing the role of the other points sharing the same cell-id. Those RRUs could represent low power points with one or more antennas each. Another example is when all the points have a similar power class with no single point having more significance than the others. The base station would then handle the signals from all RRUs in a similar manner.
An advantage of the shared cell approach compared with the classical approach is that the typically involved handover procedure between cells only needs to be invoked on a macro basis. Another advantage is that interference from Cell specific Reference Symbols (CRS) can be greatly reduced since CRS does not have to be transmitted from every point. There is also greater flexibility in coordination and scheduling among the points which means the network can avoid relying on the inflexible concept of semi-statically configured “low interference” subframes as in Rel-10. A shared cell approach also allow decoupling of the downlink (DL) with the uplink (UL) so that for example path loss based reception point selection can be performed in UL while not creating a severe interference problem for the DL, where the UE may be served by a transmission point different from the point used in the UL reception.